Handle orientation and screen size changes.
Handle orientation and screen size changes.

This prevents the activity from being destroyed and recreated when the
user rotates the display.

Closes trac #392.

prati0100/zulip-android,zulip/zulip-android,Sam1301/zulip-android,Sam1301/zulip-android,abhaymaniyar/zulip-android,saketkumar95/zulip-android,saketkumar95/zulip-android,abhaymaniyar/zulip-android,prati0100/zulip-android,vishwesh3/zulip-android